GSM (Global Systems for Mobile Communications) is the dominant world standard for 2G/2.5G wireless voice and data communications. EDGE (Enhanced Data rates for GSM Evolution) is a 3G technology that provides increased data transmission speeds of up to 384 kbit/s within the existing GSM spectrum. EDGE is an enhancement to GPRS (General Packet Radio Service) and is becoming more widely used because it effectively triples the gross data rate offered by GSM.
A major source of performance degradation in wireless telephony and data terminal products is ambient noise. Since ambient noise tends to vary significantly from environment to environment, reducing or eliminating the noise presents a challenge.
In a typical GSM communication system, speech and/or data is encoded at the source and transmitted over a network to a receiver. Upon receipt of the transmitted data, the receiver performs channel equalization and decoding steps to return the speech and/or data to a recognizable form for delivery to the user.
The channel decoder used in a GSM system is typically a forward error correction (FEC) decoder, which operates on the channel equalization output of four consecutive data transmission bursts. Conventional equalizers, such as the Viterbi equalizer, for example, do not take into account the signal-to-noise ratio (SNR) fluctuations between the data transmission bursts due to the fading nature of the channel. As such, the soft decision (SD) output of the equalizer does not reflect the input SNR. The stripping of SNR information from the SD compromises the performance of the channel decoder that follows and thus the quality of the speech and/or data that is delivered to the user. A true Maximum A posteriori Probability (MAP) equalizer accounts for the SNR, however, this type of equalizer is much more complicated than conventional equalizers. It is therefore desirable to restore the SNR information in the SD of a conventional equalizer to improve the performance of the channel decoder.